'Isildur was marching north along the east banks of the River, and... he was waylaid by the Orcs..., and almost all his folk were slain. He leaped into the waters, but the Ringslipped from his finger as he swam, and then the Orcs saw him and killed him with arrows.'

The Fellowship of the Ring, LoTR Book 1, Ch 2, The Shadow of the Past

His feet found the river bed, and heaving himself up out of the mud he floundered through the reeds to a marshy islet close to the western shore. There he rose up out of the water: only a mortal man, a small creature lost and abandoned in the wilds of Middle-earth. But to the night-eyed Orcs that lurked there on the watch he loomed up, a monstrous shadow of fear, with a piercing eye like a star. They loosed their poisoned arrows at it, and fled. Needlessly, for Isildur unarmed was pierced through heart and throat, and without a cry he fell back into the water. No trace of his body was ever found.... So passed the first victim of the malice of the masterless Ring: Isildur, second King of all the Dúnedain, lord of Arnor and Gondor, and in that age of the World the last.

†The sign † indicates a premature death, in battle or otherwise, though an annal of the event is not always included.

The Lord of the Rings, Appendix A, Annals of the Kings and Rulers

1The date of this event was calculated as follows: Isildur is said to have set out on the equivalent of September 5th in the Shire calendar; every month in that calendar had thirty days, with no between-month days (such as Lithedays) between September and October; and the attack occurred on the 30th day of the journey: